This invention relates to power supplies for use with electric and electronic circuits, and more particularly to an improved fault protection means for the secondary circuits of power supplies that use a ferro-resonant transformer. Even more particularly, the invention relates to a means of providing fault protection that does not require the proper operation of any of the secondary circuits in order for the fault protection to operate, does not use fuses or circuit breakers, provides electrical isolation between the secondary circuits and the fault detecting circuit, and can detect a fault on any number of secondary circuits.
An electrical or electronic power supply converts power from an available power source, having voltage and current levels at readily available values, to voltage and current levels having specified values needed for the proper operation of the particular electrical circuit for which the power supply was designed. For example, a power supply may typically convert the standard "line" voltage of 110 volts a.c., 50 or 60 HZ, commonly available at the electrical recepticles of most residential and commercial buildings, to a needed level, such as 5 or 12 volts d.c., having a specified polarity and current capability. The principle component used in most power supplies is the transformer. The transformer has a primary winding that is coupled through primary circuits directly to the line voltage or power source. The transformer also has one or more secondary windings, coupled to the primary circuit only through a magnetic circuit, that provide through suitable secondary circuits the specified voltage and current.
A fault in the secondary circuits of a power supply can be defined as any condition that causes excessive current to be drawn from the power source, which excessive current will likely cause damage to occur to some portion of the secondary circuits or to the primary circuit. The fault may be caused, for example, by a direct electrical short between the positive and negative voltage lines of the secondary circuit; or the fault may be caused by the failure of some component which, while not causing a direct short, causes an excessive amount of current to be drawn from the power source.
Good design practice dictates that the power supply be protected against fault conditions. Such protection not only insures the reliability of the equipment using the power supply, but also complies with the requirements of many governmental agencies, and independent testing laboratories (such as Underwriter's Laboratories, or U.L.) that qualify equipment for safety. Such protection is required to prevent the fault condition from creating an unsafe condition, e.g., a fire or a hazard to operating personnel. Hence, such protection typically consists of some mechanism that automatically disconnects the power supply from the fault, or removes the source of power (e.g., line voltage) from the power supply.
Many methods are known in the prior art that provide fault protection. Exemplary of these prior art methods are such techniques as placing fuses or circuit breakers on the primary side or the secondary side of the transformer; or designing electronic circuitry that monitors the secondary circuits and, when a fault is detected, disconnects the faulty circuitry or removes the power source (line voltage) from the primary.
Unfortunately, all of the fault protection methods of the prior art suffer from one or more inherent disadvantages. For example, it is difficult to select the size of a fuse or circuit breaker that will open fast enough to provide protection in the event of a direct short fault condition while not opening under normal momentary overload conditions of the circuit being protected. Also, some governmental agencies and independent laboratories have a maximum size on the fuse that may be used in a secondary circuit. Not only does this size vary between governmental agencies and independent laboratories, but quite often it is below the normal operating current of the secondary circuit. Thus, in some cases, a fuse may be used in the secondary circuit, only if the equipment is sold in certain areas. In other areas, a different design may be required for the power supply.
Electronic monitoring circuitry on the secondary circuits, such as "current limiting", are also commonly used in the prior art as fault protection methods. These monitoring techniques are well understood by those knowledgeable in power supply art and will not be explained here. They all suffer from the inherent disadvantage, however, of monitoring for a fault condition at some point within the secondary circuit. If a fault occurs beyond that point, i.e., away from the power supply, they can take action to protect the equipment and the power supply. However, should the fault occur inside of the monitoring point, e.g., a short across the secondary winding of the transformer, no protective action is taken.
Other electronic monitoring circuits are known in the art that monitor the current in the secondary circuits and, upon detecting a fault in a particular secondary circuit, disconnect the primary from its source of power, thereby protecting all of the secondary circuits. Unfortunately, such monitoring circuits typically require a direct electrical path between the primary and secondary circuits, and many governmental agencies and independent laboratories require electrical isolation between the primary and secondary circuits. While such electrical isolation can be provided by a variety of techniques, e.g., photo transistor/photo diode isolators or isolation transfomers, such techniques unduly add to the cost and complexity of the power supply.
Further, all of the fault protection devices known in the art, except circuit breakers or fuses in the primary, protect only the secondary circuit that is being monitored. Thus, if the transformer used in the power supply has more than one secondary circuit, as many do, the protection circuitry must be duplicated for each secondary circuit, again adding to the cost and complexity of the power supply.
Thus, it is evident that a need exists in the art for an improved method of fault protection for the secondary circuits of a power supply. Advantageously, the present invention eliminates all the aforementioned disadvantages of the prior art for power supplies that use a ferro-resonant transformer.